(663e) Carbon Sequestration Potential of Stabilized Hydrochar from Cattle Manure

Hydrothermal carbonization (HTC) is a thermochemical treatment process with applications to upgrade or convert diverse biomass feedstocks to a carbonaceous solid product. Recent discoveries in soil science and climate mitigation suggest that processes such as HTC may also provide a means to produce carbonized materials for long-term carbon sequestration in soil. Carbonized organic matter produced by fires has been found to be extremely resilient to natural decomposition and can help improve soil health and fertility under some conditions. These materials, often referred to as pyrogenic carbon, can act as a means for long-term carbon sequestration by first allowing plants to pull carbon from the air via photosynthesis, then stabilizing the plant carbon as char and storing it in soil. Thus engineers are seeking scalable methods to produce similarly stable soil amendments from organic matter for carbon sequestration. Pyrolysis, a traditional process used to make charcoal, is one such method. Pyrolysis has been demonstrated to create pyrogenic carbon, termed biochar, for use in soil. Biochar has been produced by pyrolysis of diverse feedstocks, including wood, corn stover, food waste, municipal solid waste, biosolids, and manure. In pyrolysis, the biomass is slowly heated in an inert gaseous environment to temperatures between approximately 400 and 600 °C. When applied to biomass with high moisture content, a significant amount of energy is consumed by first evaporating water to dry the biomass before the temperature reaches reaction temperature. Thus, pyrolysis is inefficient when applied to wet biomass feedstocks. On the other hand, HTC is performed in an environment of liquid water, obviating the requirement for drying.

HTC excels at converting wet feedstocks into hydrochar, a carbonaceous material with some chemical and physical similarity to biochar but is somewhat less stable in soil. When placed in soils, hydrochar has been found to release much of its carbon to the atmosphere. Thus, we seek to produce a pyrogenic carbon by way of HTC but with stability of the biochar produced by pyrolysis. The research presented here aims to address the concerns surrounding hydrochar’s limited soil stability and proposes the use of secondary processing to further stabilize hydrochar and wet agricultural waste for sequestering carbon.

We use HTC as the primary process step for thermochemical conversion of cow manure into hydrochar, and evaluate three secondary treatments for the potential to produce a stable char. Two oxidation processes were evaluated, including treatment in weak and strong chemical oxidizers (hydrogen peroxide and potassium dichromate, respectively.) The goal of these treatments is to oxidize the relatively labile carbonaceous materials, leaving behind only relatively inert material which might resist rapid decomposition in soils. Treatment with hydrogen peroxide was found to decrease the hydrochar’s inherent soil stability and treatment with potassium dichromate provided a minimal increase to its carbon sequestration potential.

We also evaluated pyrolysis for the potential to produce a stable carbon. The hydrochar produced by HTC is substantially hydrophobic and does not suffer the inefficiency of moist biomass when subjected to pyrolysis. The biochar produced from pyrolysis of hydrochar exhibited substantial improvements in markers for increased soil stability, including reduced oxygen and hydrogen content. In fact, the biochar produced from pyrolysis of hydrochar exhibits a higher carbon content as well as a higher carbon yield relative to the biochar produced from direct pyrolysis of manure. soil sequestration properties over biochar derived from pyrolysis of cattle manure. We offer several suggestions of how this two-step process can be utilized at scale for a significant impact of carbon sequestration.